CHP plants that meet the requirements of the CHPQA scheme qualify for a number of valuable benefits. Alan Hunt, product specialist for ABB Measurement & Analytics in the UK, explains how measuring CHP plant performance through good instrumentation is the key to gaining them.

In today’s energy climate, it’s easy to see why Combined Heat and Power (CHP) plants are so attractive. Essentially, CHP plants generate electricity while capturing the heat produced during the process for other uses, such as industrial production or district heating schemes. Compare this with conventional electricity production where vast amounts of heat are simply released to the atmosphere via cooling towers, wasting up to two thirds of the energy consumed.

By using this otherwise wasted heat, CHP plants can be 80% efficient, compared to around 50% for most UK gas fired power stations.

This huge gain in efficiency has led the government to encourage the installation and operation of good quality CHP projects by offering various benefits.

The Combined Heat and Power Quality Assurance (CHPQA) scheme is a voluntary programme that assesses the energy and environmental performance of CHP projects. Operators that can show their plant is eligible can currently benefit from the Climate Change Levy Exemption, Enhanced Capital Allowances and exemption from business rating for CHP plant and machinery.

Proving eligibility

To qualify for Climate Change Levy exemption on the gas used to fuel CHP schemes, operators must achieve certain metering criteria. These can be divided into two scheme types – those below 2MWe and those above 2MWe.

For schemes above 2 MWe, operators need to be able to measure the gas used, the heat supplied and the electricity generated. The gas meters must be pressure and temperature compensated and the heat meters must use a water meter to measure actual flow.

Schemes below 2 MWe can be subdivided into those that have heat dumps and those that do not.

CHP Schemes less than 2MWe with heat dumps require various combinations of gas, electricity and heat metering. If the unit is less than 500kWe and is on the approved CHPQA CHP list, then the gas consumption may be calculated by using the CHP units’ electrical efficiency as stated on the list.

If the unit is greater than 500 kWe or below 500kWe but not on the approved CHP list, then gas metering is required and the meter must be pressure and temperature compensated. The gas meter must be of a quality specified by S.I. 1983/684 (Statutory Instruments 1983 No. 684, Gas (Meters) Regulations 1983).

CHP units will normally supply heat as either hot water or steam. For measurement of steam mass flow and energy content, meters with an overall uncertainty of ±2.0% of span (full-scale) are required. For measurement of hot water, where stand-alone commercially available heat meters are used these must be manufactured to metrological Class 2 or Class 3 as defined in BS EN 1414–1997 (~ ±1.5%). For fluid temperature measurements, matched pairs of platinum resistance thermometers to BS 1041-3:1989 and BS EN 60751:1996/IEC 60751:1983 are preferred.

For CHP Schemes less than 2MWe without heat dumps, the measurement of gas and electricity is the same as for units that have heat dumps installed. The difference comes in the measurement of heat.

Existing schemes below 2MWe with no heat rejection facility are not required to meter heat outputs, as the heat output may be calculated using the design Heat / Power ratio for the unit.

For new schemes or those which supply heat in the form of steam, heat must be metered in the same way as units greater than 2MWe.

Oil burned as either a main or a standby fuel will need to be monitored and recorded. Oil products are usually sold by the litre and this will require a volume flow meter with an uncertainty no greater than ±1%.

CHPQA is an annual certification process. At the end of each calendar year, a designated Responsible Person must compile the CHP scheme energy data monitored over the previous year; assess the scheme’s performance and submit this data to CHPQA on the appropriate forms for validation.

Good boiler operation is key

Optimum boiler performance is key to an efficient CHP plant. To ensure you are only generating what you need, measure the demand and compare it with what the boiler is generating. This will help identify whether steam is being lost before it reaches the place where it’s needed most.

It’s also important to optimise the combustion process by monitoring the flue gases. Careful monitoring will help strike a balance between supplying too much air, which carries heat away up the flue, and insufficient air, resulting in incomplete combustion.

It is also important to ensure that boiler duty is at optimum efficiency. For example, don’t use two boilers at 30 percent output if you can run one at 60 – 70% output. Checking that your instrumentation is up to scratch should also be a priority. Modern instruments are typically more robust and more accurate, are easier to maintain, and are less prone to drift.

The challenge of heat

The measurement of electricity and gas flow is relatively straightforward. However, because it depends on the flow of water and in particular steam, the major challenge in CHP schemes is the accurate measurement of heat.

The critical measure of the energy flowing round a boiler system is the mass of the steam. It takes ten times the energy to create 1 m3 of steam at 10 bar than at 1 bar, yet the volume is the same.

This has traditionally been the domain of differential pressure meters such as orifice plates. The disadvantage with these is that they require a lot of ancillary equipment to produce mass readings for steam, introducing an extra maintenance burden and increasing operational costs.

A better alternative is vortex and swirl meters. With virtually no maintenance requirements, they offer an accuracy of 0.5 percent over the entire flow range, compared to orifice plates which offer an accuracy of only two percent of the upper flow range. Their effective measurement range is also up to ten times greater than that of orifice plates.

Vortex meters rely on eddies or vortices formed by the flow of fluid around an obstruction, or ‘shedder’. The meters measure the frequency of the eddies as the fluid passes the shedder. By contrast, swirl meters use static veins at the meter’s entrance to induce the fluid to rotate. This automatically sets up a secondary helical rotation, the frequency of which is measured by the flow meter.

The frequencies of the vortex street in a vortex meter and of the secondary rotation in a swirl meter are each directly proportional to the volumetric flow rate of the fluid. There is no need to compensate for changes in pressure, temperature or density – only the temperature of the steam is needed to calculate the mass flow.

Swirl meters can also be fitted almost anywhere, making them ideal for retrofitting to existing steam systems. Compared to the vast majority of flow meters that need to be positioned downstream from any sources of flow turbulence, they need just two or three diameters in most applications, greatly reducing the installation space required.

Don’t forget service

Even the most accurate and sophisticated meter will not continue to perform well if it does not receive the correct servicing at the appropriate time. Reputable manufacturers should be able to help by offering a service programme tailor-made to the instrument and its application.

Other services may be designed to maximise instrument performance. These could include fully-traceable verification of flow transmitters, certifiable inspection of primary flow devices, assessment of the device’s installation, design of optimum calibration schedules for the unit and upgrade services for measurement devices and installation and commissioning of new devices

Vendors may also be able to offer a number of other services. Some of these will be directly applicable to achieving the benefits available through the CHPQA scheme. Examples could include verification of the F4 self-assessment form; verification of measurement uncertainties; improvement of qualifying power output; CHPQA audit support services; life cycle services to maintain CHP performance and support for the annual self-assessment process.

The CHPQA scheme is a major encouragement to CHP plant operators – monitoring the performance of such plants by efficient, accurate meters, backed up with the support of innovative vendors such as ABB, will ensure its valuable benefits help make a difference to their business and to the environment.